Magnetic resonance imaging (“MRI”) is a well known, highly useful, non-invasive technique for diagnosing abnormalities in biological tissue. MRI can detect abnormalities which are difficult or impossible to detect by other techniques, without the use of x-rays or invasive surgical procedures.
Magnetic resonance imaging uses changes in the angular momentum or spin of the atomic nuclei of certain elements within body tissue in a static magnetic field after excitation by radio frequency energy, to derive images containing useful information concerning the condition of the tissue. During a magnetic resonance imaging procedure, the patient is inserted into an imaging volume of a magnet that generates a static magnetic field. Gradient coils are provided within the imaging volume to generate time-varying magnetic fields along the x, y, z axis within the imaging volume, as well. Within the imaging volume, the net vector of the angular momentum or spin of nuclei of elements containing an odd number of protons or neutrons tends to align with the direction of the magnetic field. The spins of the nuclei then turn or “precess” around the direction of the applied primary magnetic field.
Exciting the nuclei of the tissue within the imaging volume by radio frequency energy at the resonant or Larmor frequency shifts the spins out of alignment with the applied magnetic field and into phase with each other. After application of the radio frequency energy has ended, the spins of the nuclei relax and return to their original spin states. As the spins relax, the nuclei emit small radio frequency signals, referred to as magnetic resonance (“MR”) signals, at the resonant or Larmor frequency, which are detected by a radio frequency antenna tuned to that frequency. The gradient magnetic fields applied during the pulse sequence spatially encode the MR signals emitted by the nuclei. After the cessation of the application of radio frequency waves, the precessing spins gradually drift out of phase with one another and back into alignment with the direction of the applied magnetic field. This causes the MR signals emitted by the nuclei to decay. The MR signals are detected by a radio frequency receiving antenna positioned within the imaging volume proximate the patient and are amplified, digitized and processed by the MRI system. Hydrogen is the most commonly detected element because it is the most abundant nuclei in the human body and emits the strongest MR signal.
The relaxation rate of the MR signals varies for different types of tissue, including injured or diseased tissue, such as cancerous tissue. The MR signals are analyzed by known mathematical techniques to provide information about the environment, as well as the concentrations, of nuclei of interest at various locations within the patient's body may be determined. This information is typically displayed as an image with varying intensities that are a function of the concentration and environment of the nuclei of interest.
A magnetic resonance imaging procedure typically comprises one or more image scanning sequences, each of which comprises a precisely timed and orchestrated series of pulses of radio-frequency energy, and variation of the three orthogonal magnetic field gradients and the data sampling window. Each scanning sequence is defined by a pulse sequence, which is a series of values for parameters corresponding to particular characteristics of the scanning sequence and the resulting MR images. An image is derived from many repetitions of the pulse sequence, where small changes are typically introduced in the phase encoding gradient of the pulse sequence parameters between repetitions to provide additional spatial encoding. Other changes to the pulse sequences may be introduced, as well. Typical parameters defined by the pulse sequence include TR time, TE time, alignment of the slice axis, slice thickness, slice location, field of view, oblique angle and resolution.
Typically, the operation of the MRI system is controlled by an MR technologist outside the scanning room. The technologist sits at a console with a monitor and uses a mouse or keyboard to click on or type in a limited set of options in a menu driven program. The technologist may select a particular predetermined pulse sequence or may select values for particular parameter of the pulse sequence from a list, based on the portion of the body to be scanned and the instructions of a doctor. When one scanning sequence or series of scanning sequences is completed, the technologist can enter a pulse sequence for another scanning sequence that can then be initiated.
However, it can take many minutes for a technologist to set up a scanning sequence. If used during surgery, the surgeon must communicate with the technician in a separate room to request a particular scanning sequence. The surgeon must then wait for the scanning sequence to be set up by the technologist, conducted and processed to yield an image. This can be a time-consuming, inefficient process. Many minutes may elapse before the surgeon has the needed image.
In U.S. Pat. No. 6,400,157 B1, assigned to the assignee of the present invention and incorporated by reference herein, a hand operated input device, such as a mouse, is used to change the value of parameters of a pulse sequence of an MRI scanning procedure to change characteristics of the resulting image. The control buttons on the mouse may be used to select one of several modes of operation of the mouse. Different imaging parameters may be changed in each mode. The use of multiple modes enables a large number of parameters to be varied with a few input portions of the mouse. For example, the values of parameters corresponding to TR time, TE time, slice axis, slice thickness, slice location, field of view, oblique angle and resolution may all be controlled through a mouse with three control buttons and a roller ball, through selection of particular buttons and movement of the mouse. The mouse may be operated by a surgeon, or other medical personnel to conduct MRI with the desired parameters during a medical procedure. The oblique angle of the image slice may be changed after entering an oblique angle mode by moving the mouse to the right or left to cause a corresponding rotation of the oblique angle of the image slice about an axis, to the right or left. A slice cursor can also be dragged by the mouse to a desired oblique angle.
MRI can be of great benefit during a surgical procedure to locate diseased tissue and to provide current images of the site of the procedure to the surgeon. Presently, however, MRI has been used predominantly for pre-operative and post-operative imaging. MRI has had limited surgical use including MRI-guided fine-needle aspiration cytology and MRI-guided stereotactic, neurosurgery. MRI can also be used to follow and guide the advance of instruments, such as catheters, to a site of interest, for example, U.S. Pat. No. 5,647,361, assigned to the assignee of the present invention.
MRI systems are available with imaging volumes large enough to conduct surgery. The magnet of the MRI system may be large enough to contain an entire surgical team. U.S. Pat. No. 6,201,394 B1, which issued on Mar. 31, 2001 from U.S. Ser. No. 08/975,913, which was filed on Nov. 21, 1997, discloses an MRI system wherein a surgical procedure may be performed within the imaging volume of the system. The system disclosed can encompass part of or an entire room. U.S. Pat. No. 6,023,165, which issued on Feb. 8, 2001 from U.S. Ser. No. 07/993,072, filed on Dec. 18, 1992, also discloses open MRI systems appropriate for conducting surgical procedures. U.S. Pat. No. 6,201,394 B1 and U.S. Pat. No. 6,023,165 are assigned to the assignee of the present invention and are incorporated by reference herein. The Quad 7000 and Quad 12000 Open MRI systems available from the FONAR Corporation, Melville, N.Y., are suitable for performing surgery and other medical procedures, as well.
When an abnormal region of biological tissue, such as a tumor, is discovered by non-invasive means, a diagnosis of the condition of the tissue is typically required in order to determine the appropriate treatment. This requires that an adequate sample of tissue be removed from the patient for histopathological analysis. Common methods for obtaining tissue samples include fine needle aspiration biopsy and large needle core biopsy.
To accurately advance the needle to the site of interest through body tissue, it would be advantageous for the surgeon to be able to see the entire length of the needle within the tissue, through an imaging modality. It has been proposed to guide surgical procedures by magnetic resource imaging (“MRI”). See, for example, U.S. Pat. No. 5,647,361 assigned to the assignee of the present invention. For MRI to assist a surgeon in guiding the needle to the site of interest through body tissue, it would be advantageous to be able to align the orientation of the image slice with the orientation of the needle so that the axis of the needle lies in the plane of the MRI image.
It has been proposed to map the coordinates of the needle by optical feedback, feed the coordinates to an MRI system and cause the slice to be aligned with the axis of the needle based on those coordinates. Such a procedure is complex.